The present invention relates to a rotary-wing aircraft, and more particularly to a rotary wing transmission gearbox system which provides variable speeds to facilitate high speed and low speed flight profiles.
The forward airspeed of a conventional rotary wing aircraft is limited by a number of factors. Among these is the tendency of the retreating blade to stall at high forward airspeeds. As the forward airspeed increases, the airflow velocity across the retreating blade slows such that the blade may approach a stall condition. In contrast, the airflow velocity across the advancing blade increases with increasing forward speed. Dissymmetry of lift is thereby generated as forward air speed increases.
This dissymmetry of lift may create an unstable condition if not equalized across the advancing and retreating sectors of the rotor disc. Typically, blade flapping and feathering are utilized to substantially equalize the lift.
However, as the forward airspeed is increased beyond a given point for a given rotor rpm, the flapping and feathering action eventually becomes inadequate to maintain substantial equality of lift over the rotor disc. At this point, reverse airflow across the retreating sector creates negative lift and, depending on the forward speed, creates a stalling or negative lift condition that travels outwardly across the blade as airspeed increases. Conventional rotors must be operated at airspeeds lower than those which cause reverse airflow across a substantial part of the retreating blade and at an rpm low enough to alleviate any potential compressibility Mach number problems at the tip of the advancing blade. This has effectively limited forward airspeeds of conventional helicopters to approximately 180 knots.
A rotary wing aircraft with a coaxial counter-rotating rigid rotor system is capable of higher speeds as compared to conventional single rotor helicopters partly due to the balance of lift between the advancing sides of the main rotor blades on the upper and lower rotor systems. In addition, the retreating side of the rotor discs are also generally free from classic retreating blade stall due to offloading of the retreating disc sector with increasing airspeed to obtain roll equilibrium by balancing the net effects of the equal and opposite moments produced by the advancing sectors of the upper and lower counter-rotating rotor systems. To still further increase airspeed, a compound rotary wing aircraft may incorporate supplemental translational thrust.
In high speed flight, the main rotor system may be unloaded from the turbojets, and rotor RPM may be controlled by adjusting collective pitch. For any rotary-wing aircraft, increasing collective pitch slows the rotational speed and decreasing collective pitch increases rotational speed. For a rotary wing aircraft in a high speed flight profile, however, rotor RPM needs to be decreased to prevent the rotor blade tips on the advancing sides of the rotor discs from entering a supersonic region as aircraft airspeed increases. Thus, as forward airspeed increases, collective pitch must be increased to prevent the rotor RPM from increasing to an undesirable level. However, as the forward airspeed is increased beyond a given point for a given rotor rpm, adjusting collective pitch eventually becomes inadequate.
The aerodynamics of high-speed rotary wing aircraft show a noticeable benefit by reducing rotor RPM in high speed cruise flight. The RPM reduction from a hover profile to a high speed flight profile is typically on the order of about 30%. Such a reduction at the engine, however, may cause problems with auxiliary systems, engine operation and available power while, a rotary-wing aircraft, which always operates at a relatively low rotor RPM, may present penalties in rotor and transmission weight, as well as maneuverability constraints. Thus, there is a need for a rotary-wing transmission gearbox system, which provides variable rotor system speeds.
Accordingly, it is desirable to provide a variable speed gearbox system for a rotary-wing aircraft that provides a “high rotor speed mode” and “low rotor speed mode” to maximize aircraft performance during a hover flight profile and a high speed cruise flight profile, respectively.